Abstract
A-Site doping with alkali ions, and/or metal substitution at the B and B′-sites, are among the key strategies in the innovative development of A2BB′X6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A2AgRhCl6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A2AgRhCl6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs2AgBiX6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A2AgRhCl6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57–0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0–4.5 eV, enabled us to demonstrate that A2AgRhCl6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs2AgRhCl6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable.
Highlights
Cs2InAgCl6 has a direct bandgap of 3.3 eV, in which the first onset of optical absorption was observed at 380 nm, with a second absorption at 585 nm (Volonakis et al, 2017); this is not suitable for application in a solar cell because the Shockley– Queisser (S-Q) limit suggests that the maximum theoretical efficiency of a solar cell can be achieved with materials that, among other properties, exhibit direct bandgaps between 1.1 and 1.4 eV (Rühle, 2016)
The use of smaller k-grid has some effect on the height of the peak, but not on the position of its occurrence (Figure 4A vs. Figure 4C). These results suggest that decreasing the size of the alkali metal ion in the lattice increases the high frequency response behavior, but the nature of the transitions involved between the valence band maximum (VBM) and conduction band minimum (CBM) is of similar character
Because of the appreciable inconsistency between the onsets of optical absorption in the dielectric spectra calculated using SCAN-rVV10 and density functional perturbation theory (DFPT)/PBEsol, we extended our calculations to compute the bandgap of the studied systems using quasiparticle G0W0 and GW0 methods (Hedin, 1965; Van schilfgaarde et al, 2006; Vienna Ab initio Simulation Package (VASP), 2020a), based on ManyBody Perturbation Theory (MBPT), where G0 is one-particle Green’s function, W0 is the screened Coulomb interaction, and GW0 is the most usual step beyond single-shot GW (G0W0) to iterate the quasi-particle energies in the Greens functions
Summary
And Mechanically Stable Halide Double Perovskites are an important class of light harvesting materials for application in solar energy technology and optoelectronics (Greul et al, 2017; Matthews et al, 2017; Xiao et al, 2017; Zhao X.-G. et al, 2017; Chen et al, 2018; Lei et al, 2018; Li H. et al, 2018; Li T. et al, 2018; Luo et al, 2018; Tan et al, 2018; Xu et al, 2018; Chu et al, 2019; Zhao et al, 2019; Zhou Y. et al, 2019). The SCAN-rVV10 functional was used for the calculation of lattice properties, density of states, and electronic band structures, it cannot be combined with DFPT for the evaluation of linear response (optical) properties due to its lack of implementation in VASP 5.4 For this reason, and for comparison purpose with the DFPT/PBEsol results, a separate set of calculations was performed using the meta-GGA functional using a Γ -centered k-point mesh 12 × 12 × 12 that invoked an electronic minimization algorithm for an exact diagonalization of the matrix, in which the derivative of the cell-periodic part of the orbitals w.r.t. k, |∇kunk>, was calculated using finite differences given by Equation (1) (VASP, 2020c),. In all calculations referred to above, the SCAN-rVV10 optimized geometries of A2AgRhCl6 were used
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